Chapter V (2)

1. Side view of the adult skull, showing the several bones united by suture; 2. side view of the fœtal skull, showing the bones imperfectly ossified, separated to some extent from each other, the interspace being occupied by membrane. The small size of the face compared with that of the cranium is strikingly apparent.]

63. These bones are firmly united together. The union of bones is technically called an _articulation_ or _joint_. All joints are either immoveable or moveable. The union of the bones of the cranium affords an example of an immoveable articulation. Prominences and indentations, like the teeth of a saw, are formed in the margins of the contiguous bones (figs. XXXVIII. and XL.). At these inequalities of surface, which are exactly adapted to each other (figs. XXXVIII. and XL.), the two bones are in immediate apposition in such a manner as to preclude the possibility of motion, and even to render the separation extremely difficult. This mode of articulation is termed a _suture_. There are certain advantages in constructing the cranium of several distinct bones, and in uniting them in this peculiar mode. 1. The walls of the vault are stronger than they could have been had they been formed of a single piece. 2. In the fœtus, the bones are at some distance from each other (fig. XLII.); at birth, they yield and overlap one another; and in this manner they conduce to the security and ease of that event. 3. Minute vessels pass abundantly and securely through the interstices of the sutures to and from the interior of the cranium; in this manner, a free communication is established between the vessels within and without this cavity. 4. It is probable that the shock produced by external violence is diminished in consequence of the interruption of the vibration occasioned by the suture; it is certain that fracture is prevented by it from extending as far as it would do in one continued bony substance.

Section of the skull. 1. Cavity of the cranium occupied by the brain; 2. cut edge of the bones of the cranium, showing the two tables of compact bone and the intervening spongy texture called diploë.]

64. The vault of the cranium forms a cavity which contains the brain (fig. XLIII. and XLVIII.) The size of this cavity is invariably proportioned to that of the organ it lodges and protects. The form and magnitude of the cavity, and consequently the shape and size of the cranium, depend upon the brain, and not of the brain upon the cranium. The soft parts model and adapt to themselves the hard, and not the hard the soft. The formation of the brain in the fœtus is anterior to that of the case which ultimately contains it; and the hard bone is moulded upon the soft pulp, not the pulp upon the bone. At every period of life, on the inner surface of the cranium there are visible impressions made by the convolutions of the brain, and the ramifications of the arteries (figs. XXXIX. 1, 2, and XL. 2), and on its external surface are depressions occasioned by the action of the external muscles. Nor does the modifying power of the brain over the bones of the cranium terminate at birth. The formation of bone, always a slow process, is never completed until the child has attained its third or fourth year, and often not until a much later period. At this tender age, the bones, which in advanced life are hard and rigid, are comparatively soft and yielding, and consequently more readily receive and retain the impression of the convolutions and of the other projecting parts of the brain, by which they are sometimes so deeply marked, that an attentive examination of the inner surface of the cranium is of itself sufficient to determine not only that some part, but to indicate the very part of the brain which has been preternaturally active. At this tender age, pressure, internal or external, general or partial, may readily change the form of the cranium. If, by a particular posture, the head of a child be unequally balanced on the spine, the brain will press more on that side of the cranium than on the other; the organ will expand in the direction to which it inclines; that portion of it will become preternaturally developed, and consequently the balance of its functions will be disturbed. An awkward way of standing or sitting, perhaps contracted inadvertently and kept up by habit; a wry neck; any cause that keeps the head constantly inclined to one side, may produce this result, examples of which and of its consequences will be given hereafter.

65. Tracing them from without inwards we see, then, that the various coverings afforded to the brain, the central organ of the animal life, seated in its vaulted cavity, are: 1. The tegument, consisting of the skin and of cellular and adipose membrane. 2. Beneath the tegument, muscles, in the forepart and at the vertex, comparatively slender and delicate; at the sides and posteriorly, thick, strong, and powerful (fig. XLIV.). 3. Beneath the muscles, a thin but dense membrane, termed the pericranium, lining the external surface of the cranial bones. 4. Beneath the pericranium, the bony substance of the cranium, consisting of two firm and hard bony plates, with a spongy, bony structure, called diploë, interposed between them (fig. XLIII. 2). 5. Immediately in contact with the inner surface of the bony substance of the cranium, and forming its internal lining, the dense and strong membrane, called the _dura mater_, not only affording a general covering to the brain, but sending firm partitions between individual portions of it (fig. XLVIII. _c._). 6. A serous membrane lining the internal surface of the dura mater, and reflected over the entire surface of the brain, termed the arachnoid tunic. 7. A thin and delicate membrane in immediate contact with the substance of the brain, descending between all its convolutions, lining all its cavities and enveloping all its fibres, called the pia mater. 8. An aqueous fluid, contained between the arachnoid membrane and the pia mater. Skin, muscle, pericranium, bone, dura mater, arachnoid membrane, pia mater, and aqueous fluid, superimposed one upon another, form, then, the covering and defence of the brain; so great is the care taken to protect this soft and tender substance.

66. The bones of the _face_ consist of fourteen, namely, the two superior maxillary or jaw-bones (fig. XXXVII. 5), the two malar or cheek bones (fig. XXXVII. 3), the two nasal bones (fig. XXXVII. 4), the two palate bones, the two ossa unguis (fig. XXXVIII. 8), the two inferior turbinated bones, the vomer (fig. XXXVII. 6), and the inferior maxilla or the lower jaw (fig. XXXVII. 7.) This irregular pile of bones is divided into the superior and inferior maxilla or jaws; the superior maxilla being the upper and immoveable portion of the face; the inferior maxilla being the lower and moveable portion of it. Besides these bones, the face contains thirty-two teeth, sixteen in each jaw. The bones of the upper jaw are united together by sutures, and the union is so firm, that they have no motion but what they possess in common with the cranium. The lower jaw is united by a distinct articulation with the cranium (figs. XXXIV. and XXXV.).

67. Besides the bones and the teguments, the face contains a number of muscles, which for the most part are small and delicate (fig. XLIV.), together with a considerable portion of adipose matter; while, as has been stated, the face and head together contain all the senses, with the exception of that of touch, which is diffused, more or less, over the entire surface of the body.

Muscles of the face.]

68. The second great division of the body, termed the TRUNK, extends from the first bone of the neck to that called the pubis in front, and to the lower end of the coccyx behind (fig. XXXIV. 2). It is subdivided into the thorax, the abdomen, and the pelvis (fig. XLV.).

69. The _thorax_ or _chest_ extends above from the first bone of the neck, by which it is connected with the head, to the diaphragm below, by which it is divided from the abdomen (figs. XLV. and LXI.). It consists partly of muscles and partly of bones; the muscular and the osseous portions being in nearly equal proportions. Both together form the walls of a cavity in which are placed the central organs of circulation and respiration (fig. LX. 2, 5). The chief boundaries of the cavity of the thorax before, behind, and at the sides, are osseous (fig. XLV.); being formed before, by the sternum or breast-bone (fig. XLV. 6); behind, by the spinal column or back bone (fig. XLV. 2, 4); and at the sides, by the ribs (fig. XLV. 7). Below, the boundary is muscular, being formed by the diaphragm (fig. LXI. 2), while above the thorax is so much contracted (fig. XLV.), that there is merely a space left for the passage of certain parts which will be noticed immediately.

70. The figure of the thorax is that of a cone, the apex being above (fig. XLV.), through the aperture of which pass the tubes that lead to the lungs and stomach, and the great blood-vessels that go to and from the heart (fig. LX.). The base of the cone is slanting, and is considerably shorter before than behind, like an oblique section of the cone (fig. XLV.).

71. The osseous portion of the walls of the thorax is formed behind by the spinal column, a range of bones common indeed to all the divisions of the trunk; for it constitutes alike the posterior boundary of the thorax, abdomen, and pelvis (fig. XLV. 2, 4, 6). It is composed of thirty distinct bones, twenty-four of which are separate and moveable on one another, and on this account are called true vertebræ (fig. XLV. 2, 4); the other five, though separate at an early period of life, are subsequently united into a single solid piece, called the sacrum (fig. XLV. 5). The bones composing this solid piece, as they admit of no motion on each other, are called false vertebræ (fig. XLV. 5). To the extremity of the sacrum is attached the last bone of the series, termed the coccyx (fig. XXXV.).

72. From above downwards, that is, from the first bone of the neck to the first bone of the sacrum, the separate bones forming the column progressively increase in size; for this column is the chief support of the weight of the head and trunk, and this weight is progressively augmenting to this point (fig. XLV. 2, 4). From the sacrum to the coccyx, the bones successively diminish in size, until, at the extremity of the coccyx, they come to a point (fig. XXXV.). The spinal column may therefore be said to consist of two pyramids united at their base (fig. XLV. 4, 5). The superior pyramid is equal in length to about one third of the height of the body, and it is this portion of the column only that is moveable.

73. The two surfaces of the spinal column, the anterior and the posterior, present a striking contrast (figs. XXXIV. and XXXV.). The anterior surface, which in its whole extent is rounded and smooth, is broad in the region of the neck, narrow in the region of the back, and again broad in the region of the loins (fig. XLV. 2, 4.). It presents three curvatures (fig. XLV. 2, 4); the convexity of that of the neck being forwards, that of the back backwards, and that of the loins again forwards (fig. XLV. 2, 4).

74. From the posterior surface of the column, which is every where irregular and rough, spring, along the median line, in regular series, strong, sharp, and pointed projections of bone (fig. XXXV.), which from being sharp and pointed, like elongated spines, are called spinous processes, and have given name to the whole chain of bones. These processes afford fixed points for the action of powerful muscles. Extending the whole length of the column, from the base of the skull to the sacrum, on each side of the spinous processes, are deep excavations, which are filled up with the powerful muscles that maintain the trunk of the body erect.

75. From the lateral surfaces of the column likewise spring short but strong projections of bone, termed transverse processes, which also give attachment to powerful muscles (fig. XLVI.).

Bones of the trunk. 1. Spinal column; 2. the seven cervical vertebræ; 3. the twelve dorsal vertebræ; 4. the five lumbar vertebræ; 5. the sacrum; 6. the sternum; 7. the true ribs; 8. the false ribs; 9. the clavicle; 10. the scapula; 11. the ilium; 12. the ischium; 13. the pubes; 14. the acetabulum; 15. the brim of the pelvis.]

76. The separate bones of the series have a kind of turning motion on each other; hence each is called a vertebra, and the name of vertebral column is often given to the entire series, as well as that of spinal column. That portion of the column which forms the neck consists of seven distinct bones, called cervical vertebræ (fig. XLV. 2); that portion which forms the back consists of twelve, called dorsal vertebræ (fig. XLV. 3); that portion which forms the loins consists of five, called lumbar vertebræ (fig. XLV. 4). Between each of these classes of vertebræ there are specific differences, but they need not be described here: all that is necessary to the present purpose is an account of the structure which is common to every vertebra.

77. By inspecting fig. XLVI. 1, it will be seen that the upper and under edges of each vertebra consist of a ring of bone, of a firm and compact texture, rendering what may be called the body of the vertebra exceedingly strong (fig. XLVI. 3). This ring of bone forms a superficial depression (fig. XLVI. 2), for the reception of a peculiar substance, immediately to be described, which is interposed between each vertebra (fig. XLVII. 2).

78. The anterior surface of the body of the vertebra is convex (fig. XLVI. 3); its posterior surface is concave (fig. XLVI. 4); from the posterior surface springs a bony arch (figs. XLVI. 5 and LIII. 1), which, together with the posterior concavity, forms an aperture of considerable magnitude (fig. XLVI. 6), a portion of the canal for the passage of the spinal cord (figs. XLVII. 3, and XLIX. 3).

View of some of the vertebræ, which by their union form the spinal column.

_a._ A vertebra of the neck; _b._ a vertebra of the back; a vertebra of the loins.

1. Ring of compact bone forming, 3, the body of the vertebra; 2. superficial depression for the reception of the intervertebral cartilage; 3. anterior surface of the body of the vertebra; 4. posterior surface; 5. bony arch; 6. opening for the passage of the spinal cord; 7. opening for the passage of the spinal nerves; 8. articulating processes by which the vertebræ are joined to each other; 9. two dorsal vertebræ united, showing the arrangement of, 10, the spinous processes; 11. a portion of a rib articulated with the side of the vertebra.]

79. Both the upper and under edges of the arch form a notch (fig. XLVI. 7.), which, together with a corresponding notch in the contiguous vertebra, completes another aperture rounder and smaller than the former, but still of considerable size (fig. XLVI. 7.), the passage of the spinal nerves (fig. XLVII. 3).

80. From both the upper and under sides of the arch proceed two short but strong projections of bone (fig. XLVI. 8.), termed the articulating processes, because it is chiefly by these processes that the vertebræ are connected together. From the beginning to the end of the series, the two upper processes of the one vertebra are united with the two lower processes of the vertebra immediately above it (fig. XLVI. 9), and around the edges of all the articulating processes are visible rough lines, which mark the places to which the articulating ligaments are attached.

81. No vertebra, except the first, rests immediately upon its contiguous vertebra (fig. XLV. 2, 4). Each is separated from its fellow by a substance of a peculiar nature interposed between them, termed the intervertebral substance (figs. XLVII. 2, and L. 2). This substance partakes partly of the nature of cartilage, and partly of that of ligament. It is composed of concentric plates, formed of oblique fibres which intersect each other in every direction. This substance, for about a quarter of an inch from its circumference towards its centre, is tough, strong, and unyielding; then it becomes softer, and is manifestly elastic; and so it continues until it approaches the centre, when it becomes pulpy, and is again inelastic. The exterior tough and unyielding matter is for the firmness of the connexion of the several vertebræ with each other; the interior softer and elastic matter is for the easy play of the vertebræ upon each other; the one for security, the other for pliancy. And the adjustment of the one to the other is such as to combine these properties in a perfect, manner. The quantity of the unyielding substance is not so great as to produce rigidity; the quantity of the elastic substance is not so great as to occasion insecurity. The firm union of its solid matter renders the entire column strong; the aggregate elasticity of its softer substance renders it springy.

1. One of the Lumbar vertebræ. 2. Intervertebral substance. 3. A portion of the spinal cord in its canal.]

82. The column is not constructed in such a manner as to admit of an equal degree of motion in every part of it. Every thing is contrived to give to that portion which belongs to the neck freedom of motion, and, on the contrary, to render that portion which belongs to the back comparatively fixed. In the neck the mechanism of the articulating processes is such as to admit of an equal degree of sliding motion forwards, backwards, and from side to side, together with a turning motion of one bone upon another; at the same time, the intervertebral substance between the several vertebræ is thick. In consequence of this mechanism, we can touch the breast with the chin, the back with the hind head, and the shoulders with the ear, while we can make the head describe more than a semicircle. But, in the back, the articulating processes are so connected as to prevent the possibility of any motion, either forwards or backwards, or any turning of one vertebra upon another, while the intervertebral substance is comparatively thin (fig. XLV. 2, 4). That portion of the column which belongs to the back is intended to afford a fixed support for the ribs, a support which is indispensable to their action in the function of respiration. In the loins, the articulating processes are so connected as to admit of a considerable degree of motion in the horizontal direction, and from side to side, and the intervertebral substance here progressively increases in thickness to the point at which the upper portion of the column is united to the sacrum (fig. XLV. 2, 4), where the degree of motion is extensive.

83. The canal for the spinal cord, formed partly by the concavity in the posterior surface of the vertebra, and partly by the arch that springs from it (fig. XLVI. 6.), is lined by a continuation of the dense and strong membrane that constitutes the internal periosteum of the cranium, the dura mater (fig. XLVIII. _c_), which, passing out of the opening in the occipital bone, called the foramen magnum (figs. XXXIX. 7, and XLIX. 3), affords a smooth covering to the canal throughout its whole extent.

_a._ The scalp, turned down.

_b._ The cut edge of the bones of the skull.

_c._ The external strong membrane of the brain (Dura Mater) suspended by a hook.

_d._ The left hemisphere of the brain, showing its convolutions.

_e._ The superior edge of the right hemisphere.

_f._ The fissure between the two hemispheres.]

1. Hemispheres of the brain proper, or cerebrum; 2. hemispheres of the smaller brain, or cerebellum; 3. spinal cord continuous with the brain, and the spinal nerves proceeding from it on each side.]

84. The spinal cord itself, continuous with the substance of the brain, passes also out of the cranium through the foramen magnum into the spinal canal (fig. XLIX. 3), enveloped in the delicate membranes that cover it, and surrounded by the aqueous fluid contained between those membranes. The size of the spinal canal, accurately adapted to that of the spinal cord, which it lodges and protects, is of considerable size, and of a triangular shape in its cervical portion (fig. XLIX. 3), smaller and rounded in its dorsal portion (fig. XLIX. 3), and again large and triangular in its lumbar portion (fig. XLIX. 3).

85. The spinal column performs several different, and apparently incompatible, offices.

First, it affords a support and buttress to other bones. It sustains the head (fig. XXXIV. 1); it is a buttress to the ribs (fig. XLVI. 7); through the sternum and ribs it is also a buttress to the superior, and through the pelvis, to the lower, extremities (fig. XXXIV. 2, 3, 4).

Secondly, it affords a support to powerful muscles, partly to those that maintain the trunk of the body in the erect posture against the force of gravitation, and partly to those that act upon the superior and inferior extremities in the varied, energetic, and sometimes long-continued movements they execute.

Thirdly, it forms one of the boundaries of the great cavities that contain the chief organs of the organic life. To the support and protection of those organs it is specially adapted; hence the surface in immediate contact with them is even and smooth; hence its different curvatures, convexities, and concavities, have all reference to their accommodation; hence in the neck it is convex (fig. XLV. 2), in order to afford a firm support to the esophagus, the wind-pipe, the aorta, and the great trunks of the venous system (fig. LX. 3, 4); in the back it is concave, in order to enlarge the space for the dilatation of the lung in the act of inspiration (figs. XLV. 3, and LX. 5); in the loins it is convex, in order to sustain and fix the loose and floating viscera of the abdomen (figs. XLV. 4, and LX. 6, 7, 8, 9); in the pelvis it is concave, in order to enlarge the space for lodging the numerous delicate and highly-important organs contained in that cavity (fig. XLV. 5).

Fourthly, it forms the osseous walls of a canal (figs. XLVI. 6, and XLVII. 3) for the lodgment and protection of the soft and tender substance of the spinal cord, one of the great central masses of the nervous system, the seat of the animal life (fig. XLIX. 3).

Fifthly, it affords in its osseous walls secure apertures for the passage of the spinal nerves (figs. XLVI. 7, and XLIX. 3), by which impressions are transmitted from the organs to the spinal cord and brain, in the function of sensation; and from the spinal cord and brain to the organs in the function of volition.

86. For the due performance of these offices, it is indispensable that it should be firm, rigid, strong, and yet to a certain extent readily flexible in every direction. By what mechanism is it endowed with these apparently incompatible properties?

87. By means of the ring of compact bone, which forms so large a part of its body (fig. XLVI. 1) it is rendered firm, rigid, and strong. By means of its numerous separate pieces, exactly adjusted to each other, and dove-tailed into one another, an increase of strength is gained, such as it would not have been possible to communicate to a single solid piece. By the same mechanism, some degree of flexibility is also obtained; each separate bone yielding to some extent, which, though slight in a single bone, becomes considerable in the twenty-four.

88. But the flexibility required is much greater than could be obtained by this expedient alone. A rigid and immoveable pile of bones, in the position of the spinal column, on which all the other parts of the body rest, and to which they are directly or indirectly attached, would necessarily have rendered all its movements stiff and mechanical; and every movement of every kind impossible, but in a given direction. That the movements of the body may be easy, free, and varied; that it may be possible to bring into play new and complex combinations of motion at any instant, with the rapidity of the changes of thought, at the command of the impulses of feeling, it is indispensable that the spinal column be flexible in every direction, forwards, backwards, and at the sides: it is equally indispensable that it be thus capable of yielding, without injuring the spinal cord; without injuring the spinal nerves; without injuring the thoracic and abdominal viscera; and without injuring the muscles of the trunk and extremities. The degree in which it possesses this power of flexibility, and the extent to which, by the cultivation of it, it is sometimes actually brought, is exemplified in the positions and contortions of the posture-master and the tumbler. It is acquired by means of the intervertebral substance, the compressible and elastic matter interposed between the several vertebræ. So compressible is this substance, that the human body is half an inch shorter in the evening than in the morning, having lost by the exertions of the day so much of its stature; yet, so elastic is this matter, that the stature lost during the day is regained by the repose of the night. The weight of the body pressing in all directions upon the spinal column; muscles, bones, cartilages, ligaments, membranes, with all their vessels and all the fluids contained in them; the weight of all these component parts of the head, trunk, and extremities, pressing, without the cessation of an instant, during all the hours of vigilance, upon the intervertebral substance, compresses it; but this weight, being taken off during the night, by the recumbent posture of the body, the intervertebral substance, in consequence of its elasticity, regains its original bulk, and of course the spinal column its original length.

89. But the flexibility acquired through the combined properties of compressibility and elasticity is exceedingly increased by the action of the pulpy and inelastic matter in the centre of the intervertebral substance; this matter serving as a pivot to the vertebræ, facilitating their motion on each other. Its effect has been compared to that of a bladder partly filled with water, placed between two trenchers; in this case, the approximation of the circumference of the two trenchers on one side, would instantly displace a portion of the water on that side, which would occupy the increasing space on the other, with the effect of facilitating the change, in every possible direction, of the position of the two trenchers in relation to each other. To this effect, however, it is indispensable that the matter immediately around this central pivot should be, not like itself, rigid and unyielding, but compressible and elastic. It is an interesting fact, that since this illustration was suggested, it has been discovered that this very arrangement is actually adopted in the animal body. In certain animals, in the very centre of their intervertebral substance, there has been actually found a bag of water, with a substance immediately surrounding the bag, so exceedingly elastic, that when the bag is cut, the fluid contained in it is projected to the height of several feet in a perpendicular stream.

90. But besides securing freedom and extent of motion, the intervertebral substance serves still another purpose, which well deserves attention.

Firmness and strength are indispensable to the fundamental offices performed by the column; and to endow it with these properties, we have seen that the external concentric layers of the intervertebral substance are exceedingly tough and that they are attached to the bodies of the vertebræ, which are composed of dense and compact bone. But than dense and compact bone, nothing can be conceived better calculated to receive and transmit a shock or jar on the application of any degree of force to the column. Yet such force must necessarily be applied to it in every direction, from many points of the body, during almost every moment of the day; and did it actually produce a corresponding shock, the consequence would be fatal: the spinal cord and brain would be inevitably killed; for the death of these tender and delicate substances may be produced by a violent jar, although not a particle of the substances themselves be touched. A blow on the head may destroy life instantaneously, by what is termed concussion; that is, by the communication of a shock to the brain through the bones of the cranium. The brain is killed; but on careful examination of the cerebral substance after death, not the slightest morbid appearance can be detected: death is occasioned merely by the jar. A special provision is made against this evil, in the structure of the bones of the cranium, by the interposition between its two compact plates of the spongy substance called diploë (fig. XLIII. 2); and this is sufficient to prevent mischief in ordinary cases. A great degree of violence applied directly to the head is not common: when it occurs it is accidental: thousands of people pass through life without ever having suffered from it on a single occasion: but every hour, in the ordinary movements of the body, and much more in the violent movements which it occasionally makes, a degree of force is applied to the spinal column, and through it transmitted to the head, such as, did it produce a proportionate shock, would inevitably and instantly destroy both spinal cord and brain. The evil is obviated partly by the elastic, and partly by the non elastic properties of the matter interposed between the several layers of compact bone. By means of the elastic property of this matter, the head rides upon the summit of the column as upon a pliant spring, while the canal of the spinal cord remains secure and uninvaded. By means of the soft and pulpy portion of this matter, the vibrations excited in the compact bone are absorbed point by point as they are produced: as many layers of this soft and pulpy substance, so many points of absorption of the tremors excited in the compact bone; so many barriers against the possibility of the transmission of a shock to the delicate nervous substance.

91. Alike admirable is the mechanism by which the separate pieces of the column are joined together. If but one of the bones were to slip off its corresponding bone, or to be displaced in any degree, incurable paralysis, followed ultimately by death, or instantaneous death, would happen; for pressure on the spinal cord in a certain part of its course is incompatible with the power of voluntary motion, and with the continuance of life for any protracted term; and in another part of its course, with the maintenance of life beyond a few moments. To prevent such consequences, so great is the strength, so perfect the attachment, so unconquerable the resistance of that portion of the intervertebral substance which surrounds the edge of the bodies of the vertebræ, that it will allow the bone itself to give way rather than yield. Yet such is the importance of security to this portion of the frame, that it is not trusted to one expedient alone, adequate as that might seem. Besides the intervertebral substance, there is another distinct provision for the articulation of the bodies of the vertebræ. Commencing at the second cervical vertebra, in its fore part, and extending the whole length of the column to the sacrum, is a powerful ligament, composed of numerous distinct longitudinal fibres (fig. L.), which are particularly expanded over the intervals between the bones occupied by the intervertebral substance (figs. L. 1, and LI. 2, 2). This ligament is termed the _common anterior vertebral_, beneath which, if it be raised from the intervertebral substance, may be seen small _decussating_ fibres, passing from the lower edge of the vertebra above, to the upper edge of the vertebra below (fig. L. 3), from which circumstance these fibres are termed _crucial_.

1. Common anterior ligament; 2. intervertebral substance. The anterior ligament is removed to exhibit (3.) the crucial fibres passing over it.]

1. Portion of the occipital bone; 2. common anterior ligament.]

92. Corresponding with the ligament on the anterior, is another on the posterior part of the spine (fig. LII. 1), which takes its origin from the foramen magnum (fig. LII. 1); descends from thence, within the vertebral canal, on the posterior surface of the bodies of the vertebra (fig. LII. 1), and extends to the sacrum. This ligament is termed the _common posterior vertebral_, which, besides adding to the strength of the union of the bodies of the vertebræ, prevents the column itself from being bent too much forward.

1. Posterior vertebral ligament.]

93. Moreover, the bony arches of the vertebræ (fig. LIII. 1) are connected by means of a substance partly ligamentous, and partly cartilaginous (fig. LIII. 2), which, while it is extremely elastic, is capable of resisting an extraordinary degree of force.

1. Arches of the vertebræ seen from within; 2. ligaments connecting them.]

94. And in the last place, the articular processes form so many distinct joints, each being furnished with all the apparatus of a moveable joint, and thus possessing the ordinary provision for the articulation of bones, in addition to the whole of the foregoing securities.

95. "In the most extensive motion of which the spinal column is capable, that of flexion, the common anterior ligament is relaxed; the fore part of the intervertebral substance is compressed, and its back part stretched; while the common posterior ligament is in a state of extension. In the _extension_ of the column the state of the ligaments is reversed; those which were extended being in their turn relaxed, while the common anterior vertebral is now put upon the stretch. In the _lateral inclination_ of the column, the intervertebral substance is compressed on that side to which the body is bent. In the _rotatory_ motion of the column, which is very limited in all the vertebræ, but more particularly in the dorsal, in consequence of their attachment to the ribs, the intervertebral substance is contorted, as are likewise all the ligaments. All the motions of the column are capable of being aided to a great extent by the motion of the pelvis upon the thighs."

96. "The number and breadth of the attachments of these bones," says an accomplished anatomist and surgeon,[4] "their firm union by ligament, the strength of their muscles, the very inconsiderable degree of motion which exists between any two of them, and lastly, the obliquity of their articular processes, especially in the dorsal and lumbar vertebræ, render dislocation of them, at least in those regions, impossible without fracture; and I much doubt whether dislocation even of the cervical vertebræ ever occurs without fracture, either through their bodies or their articular processes. The effects of each of these accidents would produce precisely the same injury to the spinal marrow, and symptoms of greater or less importance, according to the part of the spinal column that is injured. Death is the immediate consequence if the injury be above the third cervical vertebra, the necessary paralysis of the parts to which the phrenic and intercostal nerves are distributed causing respiration instantly to cease. If the injury be sustained below the fourth cervical vertebra, the diaphragm is still capable of action, and dissolution is protracted. The symptoms, in fact, are less violent in proportion as the injury to the spinal marrow is further removed from the brain; but death is the inevitable consequence, and that in every case at no very distant period."

97. So the object of the construction of the spinal column being to combine extent and freedom of motion with strength, and it being necessary to the accomplishment of this object to build up the column of separate pieces of bone, the connecting substances by which the different bones are united are constituted and disposed in such a manner as to prove absolutely stronger than the bones themselves. Such is the structure of this important portion of the human body considered as a piece of mere mechanism; but our conception of its beauty and perfection would be most inadequate if we did not bear in mind, that while the spinal column performs offices so varied and apparently so incompatible, it forms an integrant portion of a living machine: it is itself alive: every instant, blood-vessels, absorbents and nerves, are nourishing, removing, renewing, and animating every part and particle of it.

98. The anterior boundary of the thorax is formed by the bone called the sternum, or the breast-bone, which is broad and thick at its upper, and thin and elongated at its lower extremity (figs. XLV. 6, and LIV.), where it gives attachment to a cartilaginous appendix, which being pointed and somewhat like a broadsword, is called the ensiform cartilage.

Anterior view of the sternum.]

99. Its position is oblique, being near the vertebral column at the top, and distant from it at the bottom (fig. XLV. 6). Its margins are thick, and marked by seven depressions, for the reception of the cartilages of the seven true ribs (fig. LIV). Its anterior surface is immediately subjacent to the skin, and gives attachment to powerful muscles, which act on the superior extremities: its posterior surface is slightly hollowed in order to enlarge the cavity of the thorax (fig. LV.).

Posterior view of the sternum.]

100. The thorax is bounded at the sides by the ribs, which extend like so many arches between the spinal column and the sternum (fig. XLV. 7, 8). They are in number twenty-four, twelve on each side, of which the seven upper are united to the sternum by cartilage, and are called true ribs (fig. XLV. 7); the cartilages of the remaining five are united with each other and are not attached to the sternum; these are called false ribs (fig. XLV. 8): all of them are connected behind to the spinal column (fig. XXXV.).

101. The ribs successively and considerably increase in length as far as the seventh, by which the cavity they encompass is enlarged; from the seventh they successively diminish in length, and the capacity of the corresponding part of the cavity is lessened. The direction of the ribs from above downwards is oblique (fig. XLV. 7, 8). Their external or anterior surface is convex (fig. XLV. 7, 8); their internal or posterior surface is concave: by the first their strength is increased; by the second the general cavity of the thorax is enlarged (fig. XLV. 7, 8). Their upper margin is smooth and rounded, and gives attachment to a double layer of muscles, called the intercostal, placed in the intervals that separate the ribs from each other (fig. LIX.). Along the lower margin is excavated a deep groove, for the lodgment and protection of the intercostal vessels.

102. The ribs are connected with the spinal column chiefly by what is termed the _anterior ligament_ (fig. LVI. 1), which is attached to the head of the rib (fig. LVI.), and which, dividing into three portions (fig. LVI. 1), firmly unites every rib to two of the vertebræ, and to the intervertebral substance (fig. LVI. 1). This articulation is fortified by a second ligament (fig. LVI. 2), also attached to a head of the rib, termed the _interarticular_ (fig. LVI. 2), and by three others, one of which is attached on the fore part, and the two others in the back part, to the neck of the rib (fig. LVII. 1).

Ligaments connecting the ribs to the spinal column. 1. anterior ligaments; 2. interarticular ligament; 3. ligaments of the necks of the ribs.]

The cartilages of the seven superior ribs are attached to the sternum by a double layer of ligamentous fibres, termed the _anterior and the posterior ligaments of the sternum_ (fig. LVIII.). So strong are the bands which thus attach the ribs to the spinal column and the sternum, that the ribs cannot be dislocated without fracture. "Such at least is the case in experiments upon the dead body, where, though the rib be subjected to the application of force by means of an instrument best calculated to detach its head from the articulation, yet it is always broken."

1, &c. Ligaments connecting the ribs to the vertebræ behind.]

While thus firmly attached to their points of support, the ligaments, which fix them, are so disposed as to render the ribs capable of being readily moved upwards and downwards: upwards in inspiration; downwards in expiration; and it is by this alternate action that they enlarge and diminish the cavity of the thorax in the function of respiration.

Ligaments joining the cartilages of the ribs to the sternum.]

103. Such are the boundaries of the cavity of the thorax as far as its walls are solid. The interspaces between these solid portions at the sides are filled up by muscles, principally by those termed the intercostal (fig. LIX.); below, the boundary is formed by the diaphragm (fig. LXI. 2); while above, as has been already stated (69), the cavity is so contracted as only to leave an opening for the passage of certain parts to and from the chest.

A view of the muscles called _Intercostals_, filling up the spaces between the ribs.]

104. The inner surface of the walls of the thorax, in its whole extent, is lined by a serous membrane, exceedingly thin and delicate, but still firm, called the pleura. The same membrane is reflected over the organs of respiration contained in the cavity, so as to give them an external coat. The membrane itself is every where continuous, and every where the same, whether it line the containing or the contained parts; but it receives a different name as it covers the one or the other: that portion of it which lines the walls of the cavity being called the costal pleura (fig. LXI. _a_), while that which covers the organs contained in the cavity is termed the pulmonary pleura (fig. LX. 5, 1).

105. A fold of each pleura passes directly across the central part of the cavity of the thorax; extending from the spinal column to the sternum, and dividing the general cavity into two. This portion of the pleura is called the mediastinum, from its situation in the centre of the thorax, and it so completely divides the thoracic cavity into two, that the organs on one side of the chest have no communication with those of the other; so that there may be extensive disease in one cavity (for example, a large accumulation of water,) while the other may be perfectly sound.

106. The main organs contained in the cavity of the thorax are the lungs with their air tube; the heart with its great vessels; and the tube passing from the mouth to the stomach (fig. LX.).

107. The two lungs occupy the sides of the chest (fig. LX. 5). They are completely separated from each other by the membranous partition just described, the mediastinum. Between the two folds of the mediastinum, namely, in the middle of the chest, but inclining somewhat to the left side, is placed the heart, enveloped in another serous membrane, the pericardium (fig. LX. 2, 1).

108. The lungs are moulded to the cavities they fill; whence their figure is conical, the base of the cone being downwards, resting on the diaphragm (fig. LX. 5, _b_); and the apex upwards, towards the neck (fig. LX. 5).

109. That surface of each lung which corresponds to the walls of the chest is convex in its whole extent (fig. LX. 5); on the contrary, that surface which corresponds to the mediastinum is flattened (fig. LX. 5). The basis of the lung is concave, adapted to the convexity of the diaphragm on which it rests (fig. LX. 5).

110. The air-vessel of the lungs, termed the bronchus, together with the blood-vessels and nerves, enter the organ at its flattened side, not exactly in the middle, but rather towards the upper and back part. This portion is termed the root of the lung.

111. The lungs are attached to the neck by the trachea (fig. LX. 4), the continuation of which forms the bronchus; to the spinal column by the pleura, and to the heart by the pulmonary vessels (fig. LX. 3, _d_): their remaining portion is free and unattached.

112. In the living body, the lungs on each side completely fill the cavity of the chest, following passively the movements of its walls, and accurately adapting themselves to its size, whether its capacity enlarge in inspiration, or diminish in expiration, so that the external surface of the lung (the pulmonary pleura) is always in immediate contact with the lining membrane of the walls of the cavity (the costal pleura); consequently, during life, there is no cavity, the chest being always completely full.

_a._ The cut edges of the ribs, forming the lateral boundaries of the cavity of the thorax.

_b._ The diaphragm, forming the inferior boundary of the thorax, and the division between the thorax and the abdomen.

_c._ The cut edges of the abdominal muscles, turned aside, exposing the general cavity of the abdomen.

1. The cut edge of the pericardium turned aside.

2. The heart.

3. The great vessels in immediate connexion with the heart.

4. The trachea, or wind-pipe.

5. The lungs.

6. The liver.

7. The stomach.

8. The large intestine.

9. The small intestines.

10. The urinary bladder.]

113. The anterior surface of the pericardium, the bag which envelopes the heart, lies immediately behind the sternum, and the cartilages of the second, third, fourth, and fifth ribs, covered at its sides by the pleura, and firmly attached below to the diaphragm (fig. LX. 1).

114. Surrounded by its pericardium, within the mediastinum, the heart is placed nearly in the centre of the chest, but its direction is somewhat oblique, its apex being directly opposite to the interval between the fifth and sixth ribs on the left side (fig. LX. 2); while its basis is directed upwards, backwards, and towards the right (fig. LX. 2). That portion of its surface which is presented to view on opening the pericardium is convex (fig. LX. 2); but its opposite surface, namely, that which rests upon the part of the pericardium which is attached to the diaphragm, is flattened (fig. LX. 1). It is fixed in its situation partly by the pericardium and partly by the great vessels that go to and from it. But under the different states of expiration and inspiration, it accompanies, in some degree, the movements of the diaphragm; and in the varied postures of the body, the heart deviates to a certain extent from the exact position here described.

115. The second division of the trunk, the _abdomen_, is bounded above by the diaphragm (fig. LXI. 2), below by the pelvis (fig. LXI. 3), behind and at the sides by the vertebræ and muscles of the loins (fig. LXIII.), and before by the abdominal muscles (fig. LXIII. 9).

116. The organ which forms the superior boundary of the abdomen, the diaphragm (midriff), is a circular muscle, placed transversely across the trunk, nearly at its centre (fig. LXI. 2). It forms a vaulted partition between the thorax and the abdomen (fig. LXI. 2). All around its border it is fleshy (fig. LXI. 2); towards its centre it is tendinous (fig. LXI. 2); the surface towards the abdomen is concave (fig. LXI. 2); that towards the thorax convex (fig. LXI. 2); while its middle tendinous portion ascends into the thorax as high as the fourth rib (fig. LXI. 2).

View of the diaphragm. 1. Cavity of the thorax; 2. diaphragm separating the cavity of the thorax from that of the abdomen; 3. cavity of the pelvis.]

117. The diaphragm is perforated by several apertures, for the transmission of tubes and vessels, which pass reciprocally between the thorax and abdomen (fig. LXII.).

1. A separate aperture is formed to afford an exit from the thorax of the aorta, the common source of the arteries (fig. LXII. 2), and an entrance into the thorax of the thoracic duct, the tube that bears the digested aliment to the heart. 2. A little to the left of the former, there is another aperture, through which passes the esophagus or gullet (fig. LXII. 3), the tube that conveys the food from the mouth to the stomach. 3. On the right side, in the tendinous portion of the diaphragm, very carefully constructed, is a third aperture for the passage of the vena cava (fig. LXII. 4), the great vessel that returns the blood to the heart from the lower parts of the body.

View of the diaphragm with the tubes that pass through it. 1. Arch of the diaphragm; 2. the trunk of the aorta passing from the chest into the abdomen; 3. the esophagus passing from the chest through the diaphragm to the stomach; 4. the vena cava, the great vein that returns the blood to the heart from the lower parts of the body, passing from the abdomen, into the chest, in its way to the right side of the heart; 5. 6. muscles that arise in the interior of the trunk and that act upon the thigh; 5. the muscle called psoas; 6. the muscle called iliacus.]

118. The partition formed by the diaphragm between the thorax and abdomen, though complete, is moveable; for as the diaphragm descends in inspiration and ascends in expiration, it proportionally enlarges or diminishes the cavities between which it is placed; consequently, the actual magnitude of these cavities varies every moment, and the size of the one is always in the inverse ratio of that of the other.

119. Between the abdomen and the pelvis there is no separation; one cavity is directly continuous with the other (fig. LXI. 3); but along the inner surface of the expanded bones, which form a part of the lateral boundary of the abdomen, there is a prominent line, termed the brim of the pelvis (fig. XLV. 15), marking the point at which the abdomen is supposed to terminate and the pelvis to commence.

120. Behind and at the sides the walls of the abdomen are completed partly by the lumbar portion of the spinal column and partly by the lumbar muscles (fig. XLV. 4), and before by the abdominal muscles (fig. LXIII. 9).

121. The inner surface of the walls of the abdomen is lined throughout by a serous membrane, termed the peritoneum (fig. LXIII.). From the walls of the abdomen, the peritoneum is reflected upon the organs contained in the cavity, and is continued over them so as to form their external coat. The peritoneum also descends between the several organs, connecting them together, and holding them firmly in their situation; and it likewise forms numerous folds, in which are embedded the vessels and nerves that supply the organs. It secretes a serous fluid, by which its own surface and that of the organs it covers is rendered moist, polished, and glistening, and by means of which the organs glide smoothly over it, and over one another in the various movements of the body, and are in constant contact without growing together. In structure, distribution, and function, the peritoneum is thus perfectly analogous to the pleura.

122. Like the thorax, the abdomen is always completely full. When the diaphragm is in action, it contracts. When the diaphragm is in the state of contraction, the abdominal and lumbar muscles are in the state of relaxation. By the contraction of the diaphragm, the organs contained in the abdomen are pushed downwards, and the anterior and lateral walls of the cavity being at this moment in a state of relaxation, they readily yield, and, consequently, the viscera are protruded forwards and at the sides. But the abdominal and lumbar muscles in their turn contract, the diaphragm relaxing; and, consequently, the viscera, forced from the front and sides of the abdomen, are pushed upwards, together with the diaphragm, into the cavity of the thorax. A firm and uniform pressure is thus at all times maintained upon the whole contents of the abdomen: there is an exact adaptation of the containing to the contained parts, and of one organ to another. No space intervenes either between the walls of the abdomen and the organs they enclose, or between one organ and another: so that the term cavity does not denote a void or empty space, but merely the extent of the boundary within which the viscera are contained.

123. The contents of the abdomen consist of the organs which belong to the apparatus of digestion, and of those which belong to the apparatus of excretion.